The present invention relates to hulls for water craft and, more specifically, to longitudinal hull sections and the design thereof. Longitudinal hull sections for planing craft of existing art offer relatively low lift coefficients as well as modest lift/drag ratios. The present invention describes alternative longitudinal sections having higher lift coefficients whilst also giving significantly higher lift/drag ratios.
It will be appreciated that the term xe2x80x9clongitudinal sectionxe2x80x9d as used above and hereinbelow in relation to a hull is well understood in the art and refers to the section of the hull aligned to the longitudinal axis of the hull and containing the profile of the underside of the hull.
The pressure distribution along a typical longitudinal hull section of known art is shown in FIG. 1. The pressure coefficient PC attains unity at the leading edge stagnation point 1, falling away rapidly and asymptotically approaching zero at the trailing edge 2. The centre of lift is at a point approximately 30% along the chord (i.e. the wetted length) of the hull. In practice, the pressure coefficient Pc for typical longitudinal sections of hulls of the known art falls away more rapidly than shown. (This phenomenon also moves the centre of lift forwards.) This is due to the low aspect ratio and Vee entry shape of planing hulls of known prior art craft.
Water craft are also known which incorporate one or more flaps or xe2x80x9ctrim tabsxe2x80x9d at the trailing edge of the longitudinal hull section. Such flaps are generally inclined at a relatively small angle to the horizontal, as shown in U.S. Pat. Nos. 5,806,455, 5,215,029 and EP-A-0 071 763, for example. The pressure distribution for a typical longitudinal hull section embodying such a trailing edge flap is shown in FIG. 2. In this case a second pressure peak is established forward of the trailing edge at 3. This has the effect of increasing the pressure along almost the whole section, substantially increasing lift and moving the centre of lift rearwards to approximately 48% of the hull chord. However, flaps of this type normally have an appreciable chord (i.e. the length of the wetted portion of the flap) and as the pressure acts normally to the surface of the flap (apart from a small friction element acting along the surface of the flap), the increase in lift is gained at the expense of a considerable increase in drag if the flap angle (i.e. the angle of the flap relative to the horizontal) is appreciable. Additionally, most trailing edge flaps of this type do not extend over the entire width of the hull and this results in very high tip losses and uneven pressure distribution over the hull. Thus the use of such flaps is a palliative dearly bought to correct the attitude and performance of craft which would otherwise be badly balanced.
One further problem frequently experienced with planing hulls of known art is longitudinal instability at high speed one reason for which is the effect of the bow being apparently xe2x80x98suckedxe2x80x99 into an approaching wave. This latter effect causes considerable drag as the bow can only lift once sufficient displacement lift has been generated or when the wave has passed. Thus FIG. 3 shows a planing hull section of known art in which the forward section is rounded due to immersion in excess of the design value. The corresponding pressure distribution (Coefficient of Pressure, Pc vs. Chord) along the section is shown in FIG. 4. After attaining a value of unity at the stagnation point 1, the pressure coefficient Pc rapidly drops, becoming negative at 10% of the chord and only becoming positive again 35% along the chord. The situation worsens with increased curvature such that the pitching moment can become negative resulting in negative dynamic lift as the stem rises. Due to the negative lift over the curved section the lift/drag ratio of the section shown is only about ⅕th of the value for the same section at its design altitude shown in FIG. 1.
It is an aim of th present invention to avoid or minimise one or more of the foregoing disadvantages.
WO 96/20106 discloses a hull for planing or semi-planing watercraft, the hull having a lower surface and an abruptly down-swept trailing portion.
According to the present invention, such a hull is characterised in that a blending surface is provided between the lower surface and the trailing portion.
The trailing edge portion may be integrally formed in the hull. Preferably, though, the trailing edge portion is provided in the form of a flap means projecting generally downwardly from the hull. The flap means is preferably angled at less than 45 degrees to the normal to the design water plane, and may be substantially normal to the design water plane. The flap means preferably extends across the full width of the transom of the hull. The angle of the flap means is preferably fixed but may alternatively be formed and arranged to be variable.
The flap means preferably projects beyond the level of a portion of the underside of the hull immediately adjacent to the flap means, by a length or xe2x80x9cchordxe2x80x9d which is a small fraction of the length of the hull, typically by less than 1% of the full length of the hull. The chord of the flap means may advantageously be varied by sliding the flap means upwardly or downwardly along an inclined axis on which the flap means may be slidably mounted. Mechanical, electrical and/or hydraulic means may be provided for controlling this movement of the flap means.
The hull may have a nose portion comprising a forward surface extending rearwardly and downwardly from a nose of the hull towards the trailing edge portion, which forward surface is lightly cambered such that, in longitudinal section the hull, the angle of said forward surface relative to the water plane, in use of the hull, is progressively reduced along the length of said nose portion. The lightly cambered nose portion preferably blends smoothly into a rear portion of the underside of the hull which is normally immersed when the craft is moving at its designed operating speed. This rear portion of the underside of the trailing edge portion is preferably also cambered such that the angle of incidence of said underside relative to the design water plane, at the point where the underside meets the forward surface of the nose portion (at the design water plane), in longitudinal section of the hull, is very small, preferably less than two degrees, and may be one degree or less.
The cambered surface of the normally immersed underside may blend smoothly into a generally upswept trailing portion of the underside. This upswept trailing portion of the underside may be inclined to the design water plane, in use of the hull, at a positive or negative angle, depending on the type of craft in which the hull is incorporated, and the design speed and load conditions of the hull.
It will be appreciated that the term xe2x80x9cwater planexe2x80x9d as used herein refers to the planar intersection of the undisturbed water surface with the hull. The static water plane is the position of the water plane relative to the hull when the hull is at rest The planing water plane is the portion of the water plane relative to the hull at liftoff. The xe2x80x9cdesign water planexe2x80x9d is the position of the water plane relative to the hull at design conditions (i.e. when the hull is operating at its design speed).
It will further be appreciated that xe2x80x9clift-offxe2x80x9d refers to the point in time at which the craft incorporating the hull reaches its minimum planing speed.
The upswept trailing portion of the immersed underside of the hull is preferably immediately forward of the abruptly down-swept trailing edge portion of the hull.
In longitudinal section of the hull, the length or xe2x80x9cchordxe2x80x9d of the immersed section of the length of the hull, in use of the hull under design conditions, is preferably less than one tenth the total length of the hull.
As already described, conventional planing sections require that the craft presents a positive angle of attack in order to generate lift. An advantage of the hulls according to the invention is that they can operate at zero angle of attack at design speed and can be arranged to exhibit little or no attitude change throughout the speed range of the craft.
In addition, the hulls of the invention offer improved lift coefficients and two-dimensional lift/drag ratio. The increased lift coefficient results in a reduced surface area in contact with the water, which, for hulls of equal beam translates into a reduction in the mean chord of the lifting surface and, consequently, an increase in Froude number (Vs/(gxc3x97L)), where Vs is the boat velocity through water, L is the local chord, and g is accelaration due to gravity. This reduces both spray drag and wave making drag. Thus hulls according to the present invention offer considerable power savings compared to conventional hulls of identical weight and size, whilst both wash and spray are also considerably reduced. The reduced spray generation of hulls of this invention renders the fitting of spray rails and other devices designed to increase lift by deflecting the spray sheet downwards unnecessary.
A further benefit offered by the increased lift coefficient is that the craft is able to plane at substantially reduced speeds whilst the higher lift/drag ratio further reduces the power required to achieve planing speed. For propeller driven craft, the increased velocity under the hull due to propulsor slip generates a considerable increase in lift. This effect is much more marked than on conventional hulls due to the much reduced chord and the increased pressure distribution over the rear sections. Also, because of the higher lift coefficient and correspondingly lower planing speeds the propulsor slip is greater. This increases the local velocity relative to the craft so that the dynamic pressure is also increased. The additional lift generated may reduce the power required to achieve planing speed by 30% or more.